Solid solution of novel LixByGdEu (WO4)2 (B=Na, K) red phosphors: Influence of Na/K substitution on microstructures, Judd-Ofelt and luminescence properties for WLED applications

Abstract

In the present investigation, new materials of the type LixByGdEu(WO4)2 (B= Na, K) have been synthesized by solid state reaction. The influence of Li/K and Li/Na ions on the photoluminescence properties and structures are presented in detail. Rietveld refinement indicates the crystalline phase of the compounds and incorporation of doping leads to change in the interatomic distance. The SEM observations shows that all the prepared phosphors have spheroid, bi-pyramidal shape with different grain size. An Optical energy bandgap of 3.0 and 3.2 eV were observed for LixByGdEu(WO4)2 (B= Na, K) phosphors. The Photoluminescence spectra shows that the phosphors can be excited by near UV light which is relevant for lighting technology. Luminescence emission spectrum reveals the two characteristic emissions located at 613 nm (electric dipole) and 586 nm (magnetic dipole). The 5D0→7F2 electric dipole transitions are dominant, indicating that Eu3+ ion occupies the single crystallographic site without the stark splitting in the host matrix. Interestingly, doping of Na, K shows 40% enhancement of photoluminescence intensity than reported LiGdEu(WO4)2 phosphor. The critical distance of energy transfer is found be 10 Å indicating the multipole-multipole interactions. Judd Ofelt (J-O) theory is implemented to explore the luminescence behavior and local structure around activator with respect to Eu3+ ion. The JO intensity parameter trend i.e, Ω2 > Ω4 indicate that the asymmetric environment around the activator. In addition, the Cathodoluminesence study also suggests the intense red emission of phosphors at low voltage and current. The chromaticity coordinates of the phosphors are much closer to NTSC trichromatic coordinates indicating the promising red phosphors with excellent luminescence properties. The CCT values around 3500K reveals that the LixByGdEu(WO4)2 (B= Na, K) red phosphors can be used for warm white LED applications.